Recently, there have been increasing demands for higher efficiency and wider band in high-frequency amplifiers used in wireless base stations for mobile communications and the like. Known as an example of power supply circuits for increasing the efficiency of high-frequency power amplifiers is an envelope tracking power supply circuit which changes the power supply voltage applied to a high-frequency amplifier so as to follow an envelope of a wireless signal fed into the high-frequency amplifier (see the following Non Patent Literatures 1 to 4). The envelope tracking power supply circuit comprises a part including an operational amplifier which is a wideband power supply fed with a signal corresponding to an envelope of an input signal for a power amplifier and a part including a switching converter which is a narrowband power supply having an efficiency higher than that of the operational amplifier. Such an envelope tracking power supply circuit is effective when used as a power amplifier, which amplifies a signal having a higher peak power as compared with its average power, for a wireless base station in communication schemes such as WCDMA (Wideband Code Division Multiple Access) and OFDM (Orthogonal Frequency Division Multiplexing).